1. Field of the Invention
The present invention relates to a method for cleaning a substrate.
2. Description of the Related Art
Referring to FIG. 8, a conventional method for cleaning a semiconductor substrate will be described.
The conventional method for cleaning a semiconductor substrate (hereinafter, referred to as a substrates) includes the step of dipping a plurality of substrates 14, 15, and 16 into a cleaning solution 2. At this time, the plurality of substrates 14, 15, and 16 are dipped into the cleaning solution 2 through the surface thereof under the condition that the substrates 14, 15, and 16 are placed substantially parallel with each other. The cleaning solution 2 is accommodated in a cleaning tank 1. A supply port 4 is provided on the bottom of the cleaning tank 1. The cleaning solution 2 is supplied to the cleaning tank 1 through the supply port 4. In a lower portion of the cleaning tank 1, a rectifying plate 3 is provided. Because of the rectifying plate 3, a flow 5 of the cleaning solution 2 maintains a laminar flow. The top of the cleaning tank 1 is open, and the cleaning solution overflows the cleaning tank 1.
A plurality of substrates 14, 15, and 16 are arranged in a wafer carrier (not shown) and dipped into the cleaning solution 2 together with the wafer carrier. A generally used wafer carrier has a plurality of grooves at a pitch of 4.76 mm for supporting the respective plurality of substrates. Considering the thickness (0.65 mm) of each substrate, a distance (l) between the substrates arranged in the wafer carrier becomes 4.11 mm. The distance (l) is in accordance with the standard SEMI which was determined about twenty years ago. The standard SEMI has not changed since then. When a plurality of substrates are dipped into the cleaning solution 2 under the condition that many substrates are placed at a small distance from each other, the efficiency of the cleaning step can be increased. In addition, the decrease in the amount of a cleaning solution to be used, due to the miniaturization of the cleaning tank 1, lowers the cost for the cleaning step.
There is a tendency that the diameter of a substrate increases. Namely, a length (L) of a substrate measured in the dip direction becomes larger. On the other hand, a dip speed (V) has been tried to be made high so as to improve a throughput; however, a maximum speed is about 200 mm/sec. because of the restriction of members of a driving portion and a control system. The high-speed is only for the purpose of improving the throughput.
In the conventional method for cleaning, as shown in FIG. 8, the first substrate 14, the second substrate 15, and the third substrate 16 are dipped into the cleaning solution 2 in the vertical direction with respect to the surface of the cleaning solution 2 under the condition that a number of particles 9 adhere to the back face of each substrate. Most of the particles 9 adhering to the first, second, and third substrates 14, 15, and 16 which are thus soaked depart therefrom into the cleaning solution 2, and are removed away from the cleaning tank 1 through the flow 5 of the cleaning solution 2.
However, in the conventional technique, after the particles 9 adhering to the back face of the third substrate 16 depart therefrom, part of the departed particles 9 readhere to the front face of an adjacent substrate 15.
The degree of contamination on the back face of a substrate has been increased along with the automatization of an apparatus for producing a semiconductor device. This is because the back face of the substrate is likely to come into contact with a supporting stage and the like, while the substrate is being conveyed or various kinds of treatments are given to the substrate. In addition, the number of production steps is increased as the structure of the substrate becomes more complicated. Thus, the substrate has more chances to come into contact with the supporting stage and the like.
In the conventional technique, the following methods have been used for the purpose of protecting the back face of the substrate from the above-mentioned contamination: Preventing particles from adhering to the back face of the substrate as much as possible; and removing the particles adhering to the back face of the substrate as much as possible.
In the former method, while the substrate is being conveyed or is being treated, the back face of the substrate is prevented from coming into contact with other mechanical elements as much as possible or the contact area between the back face of the substrate and the other mechanical elements is made as small as possible. This method has a problem of the limit to decreasing the contact area.
In the latter method, for example, the particles on the back face of the substrate are removed with a scraper. However, in this method, the number of the production steps increases. Because of this, it is difficult to decrease the number of the particles adhering to the back face of the substrate.
The readhesion of the particles 9 during the cleaning step decreases the production yield of semiconductor devices which are formed on the front face of the substrate. In particular, even particles in the order of a submicron decreases the production yield of a minute semiconductor device. Thus, the readhesion of the particles 9 during the cleaning step causes a serious problem for the improvement of the production yield of a semiconductor device. Even under these circumstances, the mechanism of the readhesion of the particles 9 is not made clear, so that this problem has not sufficiently been overcome.